(svn r2685) -Codechange: Split the music/sound/video drivers into separate files and move them into subfolders.

This results in shorter and hopefully easier to maintain files.
Note: I had to change paths in #include statements of some unrelated files, because I added the ottd base directory to the include path (-I.)

music/null.c

@@ -0,0 +1,19 @@
+#include "stdafx.h"
+#include "openttd.h"
+#include "music/null.h"
+
+static const char* NullMidiStart(const char* const* parm) { return NULL; }
+static void NullMidiStop(void) {}
+static void NullMidiPlaySong(const char *filename) {}
+static void NullMidiStopSong(void) {}
+static bool NullMidiIsSongPlaying(void) { return true; }
+static void NullMidiSetVolume(byte vol) {}
+
+const HalMusicDriver _null_music_driver = {
+	NullMidiStart,
+	NullMidiStop,
+	NullMidiPlaySong,
+	NullMidiStopSong,
+	NullMidiIsSongPlaying,
+	NullMidiSetVolume,
+};